Sure...check out this URL and search for "singleton" to see a great discussion:
http://groups.yahoo.com/group/xpstl/

The project I'm working on now uses a lot of singletons, and I can speak from experience in saying that they are hard to test, hard to refactor, and make things too tightly coupled to "global variables" disguised as singletons.

I think the discussion on the Yahoo group mentioned above says it all.